1. Field of the Invention
This invention deals with a procedure and a device for displacement by sliding a mobile support on which accessories such as a part, a sensor or a tool can be fitted over a reference surface, such that distributed forces can be applied to this mobile support from the bottom upwards.
2. Description of the Prior Art
It is known that every displacement of an object in space is subjected to three geometric deviations from its movement yaw, pitch and roll.
Known solutions, using earlier technologies, propose the use of two slides superimposed on intersecting movements which provoke the superposition of these three geometric deviations and their interactions; moreover, this type of device is tributary to the machining tolerances and to the respective vibrations of each slide which propagate through the sensing system, leading to the existence of defects which can only partly be eliminated. However, in order to improve the performances of existing devices, large and heavy systems have been designed in order to limit the adverse effects of vibrations by the inertia of the device itself. It can be seen that these large parts also have to be accurately machined: leading to equipment which is disproportionately expensive compared with the results obtained.
Another known solution consists of displacing the mobile part relative to a surface, without it coming into contact with it, by means of an air-cushion system. In this case too, vibrations induced by these controlled air leakage systems make this solution unsuitable for some special applications. The air film always has some elasticity and forms a vibration amplifier.
Moreover, the most obvious solution consists of displacing the mobile part by sliding over a reference surface, however the contact pressure is proportional to the mass, thus eliminating all reproducibility of results. In order to be moved by sliding, any mobile part must be pulled or pushed by a force greater than that necessary to maintain it in friction displacement.
The difference in the sliding and friction forces applied to the mobile part controls its geometric position within a coordinate system. Moreover, these frictional values provoke dimensional deviations due to temperature variations; dissymmetry of the load distribution is another direct consequence which is observed and which affects the displacement quality, especially during stopping and starting of a mobile part. Finally there is a premature wear phenomenon of sliding surfaces in contact in this type of construction. Moreover, in order to displace the mobile part, it has to be subjected to as uniform movements as possible, which does not arise in practice.
Electric motor displacement systems with a shaft driving a rotating bolt in a ball screw introduce new defects in addition to the initial defects, which when accumulated make it impossible to carry out high precision measurements. For some applications, for example surface condition measurements, the dispersion of measurements on a profile trajectory can reach a value of 5 micrometers in Z, regardless of the type of sensor placed on the supporting mobile part.
For example, we can quote the case of mean values of irregularity spacings for different types of machining, and spacing (e) and depth (p) values in micrometers:
______________________________________ spacing depth ______________________________________ Lathe work 280 &lt; e &lt; 60 1.5 &lt; p &lt; 15 Grinding 180 &lt; e &lt; 7 4 &lt; p &lt; 0.5 Milling 400 &lt; e &lt; 17 3 &lt; p &lt; 4 ______________________________________
It will be noted that these values require high measuring precision, especially among the Z-axis. At the present time, displacement systems using two intersecting movement tables driven by stepping motors give measuring accuracies of the order of 5 micrometers, incompatible with technical surface irregularity depth measurements. In addition, standard ISO TC 57 uses the term micro-roughness when the depth of irregularities is less than 0,5 micrometers.
There is already a measurement device which uses a procedure corresponding to the foreword in claim 1 (United States patent U.S. Pat. No. 3,377,711). The mobile support displaces along rails by means of roller bearings and is laterally supported by jacks, on which the end of the rod is fitted with a rolling device and for which the offset is limited by roller end stops built into the support and which bear under the rails. This type of procedure does not give a high precision displacement relative to a reference surface, but simply a temporary removal of pressure on the roller bearings during support position changes, in order to avoid deformation of the guide rails which form the geometric displacement reference.